Method for treating tailings pond liquor

ABSTRACT

A treatment process for processing raw liquor obtained from a tailings pond containing residue from an alumina production facility is disclosed where the raw liquor is first contacted with a gas containing carbon dioxide (CO 2 ) to neutralize the raw liquor and to precipitate aluminum hydroxide Al(OH) 3 . The aluminum hydroxide is separated from the neutralized liquor, which is rich in bicarbonates and then concentrated using membrane separation technology to form a NaHCO 3  (sodium bicarbonate) rich retentate. This NaHCO 3  rich retentate is then subjected to a regeneration process to recover NaOH for recycle to the alumina production facility.

TECHNICAL FIELD

The present invention relates to the treatment of liquid materials having a high pH and alkalinity. More particularly, the invention relates to processes for the recovery of aluminum, regeneration of concentrated sodium hydroxide, and recycle/discharge of a clean water from liquors separated from so-called “red mud” that is generated in the Bayer process during the production of alumina (Al₂O₃). Red mud includes the solid residues, dissolved by-products and waste materials arising from Bauxite refineries.

BACKGROUND

A principle means of producing aluminum is through the mining and beneficiation of Bauxite ore to alumina (Al₂O₃) that is subsequently employed as a feedstock in an electrolytic process to obtain aluminum metal. The Bayer process is the predominant means of producing refined alumina from Bauxite ore while the Hall-Héroult process is the electrolyzing process often employed to produce metallic aluminum.

A substantial fraction of the Bauxite ore (typically 60%) remains undissolved in the milling and digestion steps of the Bayer process and must be filtered out prior to recovery of the aluminate via downstream freeze crystallization to Gibbsite (Al(OH)₃). This is accomplished using clarification, thickening and other separation/washing equipment, leading to the formation of a reddish-brown mud comprised of unreacted aluminum, silica, titanium and iron. Fe₂O₃ is the principle contributor to the red mud's color. Typically, for each ton of alumina that is produced, between one and two tons of red mud are produced (on a dry basis). This red mud must be either stored indefinitely or disposed of in an environmentally friendly manner.

The raw red mud is highly alkaline having a pH that is usually greater than 13.0. Consequently, there are substantial problems associated with its storage, including:

-   -   1. Highly caustic water and sediment presents a serious threat         to any wildlife or humans that come into contact with it,         because it can cause severe caustic burns or death.     -   2. The cost of containment is high and the land used for storage         is not available for other purposes.     -   3. The escape of caustic leachate from storage areas into local         groundwater systems is difficult to prevent and may persist long         after the deposition of red mud in the storage area has ceased.     -   4. The cost of managing and maintaining caustic red mud storage         facilities is high.     -   5. The costs associated with public liability insurance and         environmental protection and remediation bonds are high and are         likely to increase further in future.

The conventional practice to dispose of red mud has been by impoundment in waste ponds or lakes. In such impoundment lakes, the red mud settles out by gravity leaving an aqueous caustic phase (liquor) that may contain up to about 35 grams per liter of caustic soda and 8 grams per liter of alumina, which was present in the aqueous phase of the red mud slurry when it was pumped to the impoundment lake. Considerable quantities of caustic soda and alumina are also lost to occlusion in the red mud impoundment lakes. This represents a significant monetary loss. Aside from these chemical losses, the lake water itself poses a disposal problem from an environmental standpoint due to a high alkalinity. Groundwater runoff is not permissible unless the caustic lake water is neutralized.

In addition to the caustic and alumina losses and the negative environmental impact, conventional red mud impoundment lakes demand substantial land usage on a continuing basis. For example, a typical impoundment lake may occupy about 50-70 acres or more that is non-recoverable land and is useful for only about five years. In addition to the land costs, significant costs are realized in constructing and maintaining the dikes, which retain the waste lake water and red mud.

There is a need therefore for a process that will reduce the current volume of red mud tailings ponds. Our invention is directed to the treatment of the raw aqueous liquor obtained from such tailings ponds to recover the valuable soda and alumina values for recycle back to the alumina production facility. These and other advantages will become evident from the following more detailed description of the invention.

SUMMARY

Our invention is directed to processes for treating the raw aqueous liquor obtained from tailings ponds containing red mud residues from a Bauxite ore processing plant used to manufacture alumina as a raw material for various products, including aluminum metal. The raw aqueous liquor can be obtained by pump after the sludge is settled in the tailings pond. Alternately, the liquor can be provided as filtrate from any number of known mechanical dewatering processes and/or techniques, such as, vacuum/recessed/belt press, centrifuge, pan filters, and the like separation processes.

The aqueous liquor obtained from the tailings pond typically has a pH of 12 or higher, a relatively low total suspended solids content of <100 mg/L, approximately 2,500 to 5,000 mg/L of dissolved Al, and a total alkalinity ranging from 25,000 to 50,000 mg/L as calcium carbonate (CaCO₃). Using the raw liquor removed from the tailings pond, the first step in our process is neutralization of residual hydroxide alkalinity resulting in the precipitation of dissolved aluminate ion as Gibbsite, Al(OH)₃. As stated above, the raw liquor has a relatively high pH low Al concentration, which is soluble at this pH (as the aluminate ion). A high a concentration of carbonate alkalinity can foster the production of the bayerite polytype above about a 50% volume reduction. Bayerite is particularly amenable to formation in high carbonate matrices.

Another reason for neutralization of the hydroxide alkalinity of the aqueous liquor is to allow it to be effectively processed by either nanofiltration (NF) or reverse osmosis (RO) membranes in a downstream unit operation to concentrate the sodium bicarbonate (NaHCO₃) contained in the liquor. Most reverse osmosis membranes have limits with respect to pH and/or NaOH concentration due to chemical compatibility considerations associated with the housings, glues, and even the membrane layers themselves. Regarding the latter, some membranes will hydrolyze and swell at high pH, resulting in reduced rejection or outright premature failure. We have found that by operating in a more neutral range where bicarbonate exists, these issues are circumvented.

Although the aqueous liquor can be neutralized to precipitate the aluminum in solution using other common mineral acids, such a neutralization process will not result in the addition of soluble ions to the neutralized liquor that are amenable to soda regeneration. Having soluble ions in solution leaves the neutralized liquor in a carbonate form. The advantage of this is that a downstream regeneration process, as described below, will fully regenerate the carbonate form back into useable and valuable NaOH. Accordingly, our neutralization step in our process to treat aqueous liquor obtained from a tailings pond does not involve the use of mineral acids. Instead, our process utilizes a source of CO₂ as the neutralizing agent that will cause the hydroxyl alkalinity to be converted to bicarbonate as per the following reaction:

NaOH+CO₂→NaHCO₃

In turn, the bicarbonate is regenerable back into the hydroxide form via addition of lime, as described below, as per:

NaHCO₃+CaO→CaCO₃↓+NaOH

The precipitation of aluminum using CO₂ during the neutralization step is the conversion of the sodium aluminate to Gibbsite as per:

NaAl(OH)₄+CO₂→NaHCO₃+Al(OH)₃↓

We have found that the residual Al can be reduced to about 35 mg/L at a pH 7.5. This reduced pH level is sufficient for feeding the resultant aqueous liquor to the membrane concentration step discussed in detail below. Accordingly, our process uses a sufficient quantity of a CO₂ containing gas that will reduce the pH to a range of from about 5.5 to about 7.5.

Although the use of CO₂ is beneficial, we have found that the concurrent precipitation of Al presents special challenges because Al(OH)₃ is traditionally a fine and gelatinous precipitate. Moreover, the use of a gas-phase reagent source makes the application of conventional mixed-tank reactors problematic because the necessary gas-phase reactions require intense mixing with the aqueous phase, resulting in high shear rates which are typically detrimental to the production of large settleable solids. Consequently, our process employs a two-phase (gas-liquid) forced circulation crystallizer that may be employed also as an adiabatic quench evaporator where the source of CO2 is from hot flue or waste heat sources. Alternately, in cases where an ambient source of CO2 is employed a forced-circulation crystallizer that leverages gas dispersion coupled with high-flow radial or axial-flow impellers is applied. The use of a forced circulation reaction crystallizer promotes excellent mixing at reduced shear and while operating with a nominal 5-10% by volume concentration of solids, provides sufficient seed to promote growth of solids, leading to more settleable and filterable (and hence recoverable) Gibbsite. Typically, a slurry with a mean particle size (by volume) in the range of 5 to 50 μm can be produced.

The use of CO₂ is also beneficial because it is an inexpensive source that can be found in flue gas and waste heat sources from industrial processes, such as, the alumina production facility itself. Not only do waste heat sources provide a continuous source of inexpensive CO₂, these waste sources have the added benefit of supplying the heat (discussed further below).

As the CO₂ is adsorbed by the slurry, the hydroxyl alkalinity is neutralized, resulting in a drop of pH. As the aluminate ion is also neutralized and the pH drops appreciably below 9.5, this causes Al(OH)₃ to precipitate. In waste heat sources, the temperature of the neutralized liquor (bicarbonate solution) is raised to about 120° F. which helps increase the bicarbonate solubility alkalinity during the membrane concentration step.

Separation of the precipitated Al(OH)₃ can be conducted with a conventional clarifier as the concentration of the solids will be relatively low. This may be fostered with the addition of settling aids, e.g. coagulants and/or flocculants. Any settling aids known to the industrial wastewater treatment and clarification industry can be used such as polyacrylamide flocculants. A portion of the separated precipitated Al(OH)₃ can be removed via a slipstream and sent to a dewatering device for recovery and recycle of the Al(OH)₃ back to the production facility. Dewatering of the Gibbsite (Al(OH)₃) slurry may be accomplished with a vacuum drum, a recessed/belt filter press, or a centrifuge. The filtrate from the dewatering unit can be recycled back to the inlet of the clarifier while the filter cake could be placed into roll-off boxes or other easily transportable containers so that it can be sent to the production facility where it is blended with the Gibbsite produced in the Bayer precipitation step. Eventually the Gibbsite is sent to a calcination step for the final conversion to alumina (Al₂O₃).

The supernatant from the clarification step, where the Al(OH)₃ was separated from the bicarbonate solution may contain residual TSS (total suspended solids) that should be removed prior to the membrane concentration step in order to prevent plugging of the membranes. Removal of TSS can be accomplished with conventional multimedia filters, pressure leaf filters and/or cartridge, sock and/or pulse-jet tubular filters. The specific type of TSS filter system used is not critical provided that the filtration step, when needed, removes all suspended solids to achieve a silt density index of <3, which is a typical requirement for the successful use of reverse osmosis membrane separation technology.

As mentioned, we have found it beneficial to heat the neutralized bicarbonate containing liquor to stabilize the solution recovered from the clarifier. Although a preferred heat source is hot flue gas or other waste heat gas containing CO₂, in those cases were the CO₂ neutralization does not employ waste heat, a heat exchanger can be employed downstream of the filters to stabilize the bicarbonate solution during the membrane concentration step. Heating the solution to about 140° F. prevents precipitation of bicarbonate salts by maintaining the solubility of the bicarbonate even after concentration by four times through the use of membrane separation technology. If this optional heat exchanger is used, it can be heated with low to medium pressure steam to raise the liquor temperature to about 140° F., preferably in the range of from about 120 to 140° F.

Concentration of the bicarbonate solution is achieved using either nanofiltration or reverse osmosis membranes, or a combination of these. This step can achieve concentration increases of up to four times, equivalent to about 7 to 8% by weight NaOH. The filtrate (permeate) from the membrane operation is composed of clean water with about 4,000 to 5,000 mg/L (as CaCO3) of bicarbonate alkalinity, along with low levels of residual Al and oxalates that can be discharged to the plant's wastewater treatment plant. This filtrate can also be recycled back to the alumina production plant for use as process/plant water in other unit operations. The concentrate (retentate) from the membrane unit operations is then sent to a caustic regeneration step.

Because the Bayer process uses soda (NaOH) alkalinity to digest the Bauxite ore, which contains about 35-54% alumina, it is very beneficial to regenerate the concentrated bicarbonate solution from the NF/RO membranes back to soda (NaOH). Our process employs a regeneration step where the concentrated bicarbonate solution (the retentate) is reacted with lime in a forced-circulation crystallizer reaction step operating in double draw-off mode along with a clarifier and filter press satellite operation. A reactant lime slurry is prepared by mixing hydrated lime Ca(OH)₂, or alternately quicklime (CaO) or a mixture of both, with a recycled solution of the regenerated product NaOH obtained from an effluent holding tank. Using only a slip (or recycle) stream of the regenerated product NaOH solution minimizes dilution of the final NaOH product. The prepared lime slurry is then injected into a reaction crystallizer along with the concentrate (retentate) from the membrane step.

The reaction of hydrated lime with the membrane concentrate is as follows:

NaHCO₃+Ca(OH)₂→CaCO₃↓+NaOH+H₂O

And the use of quicklime would preclude the formation of water in the product as follows:

NaHCO₃+CaO→CaCO₃↓+NaOH

The precipitated calcium carbonate and the regenerated NaOH solution is separated in a clarification step. The sludge from clarification is then sent to a storage tank and is eventually subjected to a dewatering and/or filtering unit operation to produce a stable calcite material, which is the primary component found in limestone. A preferred dewatering step uses a recessed filter with the filtrate being returned to the effluent product NaOH tank. This regeneration step effectively sequesters the CO₂ and the carbonates in the original feed to an easily disposed of solid. With application of the reaction crystallizer, regeneration and recovery of the soda values would exceed 90% producing a filter cake of calcium carbonate containing 65+% solids by weight.

Our invention can best be summarized as a separate process distinct from the Bayer process used to produce alumina. As such, the Bayer process can be performed regardless of whether our process is in operation. Our stand-alone process is for treating raw tailings liquor obtained from red mud ponds or any other raw liquor having basically the same composition as the waste discharged from an alumina production facility.

In one embodiment of our process, the raw liquor is first contacted with a gas containing carbon dioxide (CO₂) to neutralize the raw liquor and to precipitate aluminum hydroxide Al(OH)₃. This precipitate of aluminum hydroxide is separated from the neutralized liquor and the neutralized liquor, rich in bicarbonates, is then concentrated using membrane separation technology to form a NaHCO₃ (sodium bicarbonate) rich retentate. This NaHCO₃ rich retentate is then subjected to a regeneration process to form a NaOH solution and CaCO₃ precipitate. The CaCO₃ is separated from the NaOH solution, which can then be returned to the alumina processing plant to be used in the Bayer process to digest the bauxite.

These and other embodiments will become more apparent from the detail description of the preferred embodiment contained below.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 schematically illustrates one possible embodiment of the raw liquor treatment process of our invention.

DETAILED DESCRIPTION

As stated, our invention presents a novel process for reclaiming waste liquor stored in tailings ponds as discharge from alumina production facilities, particularly those using the Bayer process. Our process, which can be operated continuously or batch wise, takes a highly alkaline wastewater and transforms it into useful components, including Gibbsite, soda, process water and calcium carbonate.

FIG. 1 shows an exemplarily process flow scheme for one particular embodiment of our process, but this is by no means the only possible flow scheme that achieves the goals of our invention. Alumina production facility 50 discharges red mud to tailings pond 1 where the solids in the red mud settle forming an aqueous liquor phase. Raw aqueous liquor 2 having a pH in the range of from about 11 to 13 is removed from the tailings pond continuously or batch wise, and is sent to contactor 3 where it is mixed and reacted with a CO₂ containing gas 4, preferably obtained from the alumina plant 50. Contactor 3 can be a conventional reactor with an axial-flow mixer, or ideally a forced circulation reaction crystallizer. The carbon dioxide neutralizes the raw liquor causing precipitation of aluminum hydroxide. Preferably, the amount of CO₂ added is sufficient to reduce the pH of the raw liquor to less than 7.5 and more preferably in the range from about 6 to 7. This mixture 5 of neutralized liquor and aluminum hydroxide is removed from contactor 3 and is sent to clarifier 7. Clarifier 7 can be a conventional gravity settle, optionally fitted with inclined tubes or plates to enhance settling area. Optionally, an aqueous filtrate 6 can also be added to the clarifier that is obtained from filter press 16.

The clarification step separates the neutralized liquor 8 and the precipitated aluminum hydroxide 9, which is removed and further dewatered in filter 16. The dewatered aluminum hydroxide (i.e., Gibbsite) 13 can be returned to plant 50 for further conversion into alumina. Neutralized liquor 8 can then be filtered to remove any suspended solids in filter 10, which preferably is a low-pressure recessed/belt press or centrifuge. The filtered neutralized liquor 11 can then be heated, if needed, using heat exchanger 12 to increase the temperature to at least 120° F. In some cases it is not necessary to supply external heat exchange because the temperature of gas 4 will be insufficient to raise the temperature of the liquor in order to stabilize the bicarbonates in solution. The stabilized liquor is then sent to a concentration step 13 that uses membrane separation technology, preferably nanofiltration or reverse osmosis or a combination of these membranes. Alternately, capacitive deionization, evaporation or membrane distillation may be employed. In the concentration step the bicarbonates are concentrated preferably to 4 times the original concentration and are removed as a retentate 15. The permeate 14 comprises essentially water that can be safety removed from the process and reused as process water in plant 50. Alternatively, the permeate 14 may be of sufficient quality to discharge to surface waters. Of course, this depends greatly on the regulations in the local jurisdiction.

The retentate 15 containing concentrated sodium bicarbonate is then subject to a NaOH regeneration series of unit operations as described below. The retentate 15 is fed to a reaction crystallizer 17. Lime 23 in the form of quick lime (CaO) or hydrated lime (Ca(OH)₂) is mixed with a product NaOH solution 24 in mixer 22 to form a lime slurry 18 that is also fed to crystallizer 17 independently or first mixed with the retentate 15. When quick lime is employed, this limits dilution of the regenerated NaOH due to water as shown in the reactions above. Within crystallizer 17 the reaction of the lime with the concentrated sodium bicarbonate forms a mixture of soda (NaOH) and calcium carbonate. This mixture 27 is removed and sent to clarifier 21. A product NaOH solution 26 is removed from clarifier 21 and sent to storage tank 31. As mentioned, a portion 24 of the product NaOH 32 can be used to form the lime slurry and the remainder 22 be sent to the alumina plant 50 for use in the Bayer process. The precipitated calcium carbonate 25 is removed from clarifier 21 and can be sent to an optional dewatering device 28 where the dewatered calcium carbonate 30 is removed and may be sold as a commodity calcite. The filtrate 29 is sent to product NaOH storage tank 31.

The foregoing description of the specific embodiments will so fully reveal the general nature of the invention that others can, by applying current knowledge, readily modify and/or adapt for various application such specific embodiments without departing from the generic concept, and therefore such adaptations and modifications are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation.

The means, materials, and steps for carrying out various disclosed functions may take a variety of alternative forms without departing from the invention. Thus, the expressions “means to . . . ” and “means for . . . ”, or any method step language as may be found in the specification above or the claims below, followed by a functional statement, are intended to define and cover whatever structural, physical, chemical or electrical element or structure, or whatever method step, which may now or in the future exist which carries out the recited function, whether or not precisely equivalent to the embodiment or embodiments disclosed in the specification above, i.e., other means or steps for carrying out the same function can be used; and it is intended that such expressions be given their broadest interpretation within the terms of the following claims. 

1. A process for treating raw tailings liquor collected from a red mud tailing pond discharged as waste from an alumina production facility comprising, (a) removing raw aqueous liquor from a red mud tailings pond; (b) contacting the raw liquor with CO₂ gas to precipitate aluminum hydroxide (Al(OH)₃) to produce a neutralized liquor; (c) separating the aluminum hydroxide from the neutralized liquor; (d) concentrating the neutralized liquor using membrane separation to form a sodium bicarbonate (NaHCO3) rich retentate; (e) regenerating the retentate to form a NaOH solution and a precipitated calcium carbonate (CaCO3); and (f) separating the NaOH solution from the CaCO3.
 2. The process of claim 1 further characterized in that flue gas from the aluminate production facility is used as a source of the CO₂ in step (b).
 3. The process of claim 1 further characterized in that CO₂ gas is used to heat the neutralized liquor to about 120° F. or higher.
 4. The process of claim 1 where the separated neutralized liquor from step (c) is heated to about 140° F. prior to the membrane separation.
 5. The process of claim 1 where the aluminum hydroxide separated in step (c) is recycled back to the alumina production facility for processing into Al₂O₃.
 6. A process for treating tailings liquor from an alumina production facility comprising, a) removing raw liquor from a tailings pond having a pH greater than 10; b) contacting the raw liquor with an industrial gas containing CO₂ in a quantity sufficient reduce the pH of the raw liquor to less than 8 causing precipitation of Al(OH)3 and forming a neutralized liquor; c) separating the Al(OH)3 from the neutralized liquor; d) filtering the neutralized liquor to remove suspended solids; e) contacting the filtered neutralized liquor from step (d) using a reverse osmosis membrane to form a concentrated NaHCO3 rich retentate and a permeate; f) contacting the retentate with lime in a crystallizer reactor to form a mixture of precipitated CaCO₃ and NaOH solution; g) clarifying the mixture of step (f) to form a NaOH product solution; h) recycling at least a portion of the NaOH product solution to the alumina production facility; and i) recovering the CaCO₃.
 7. The process of claim 6 where the industrial gas stream and the raw liquor are contacted in a counter-current scrubber using a recycled slurry of precipitated Al(OH)3.
 8. The process of claim 6 further comprising using the industrial gas stream to heat the neutralized liquor to at least 120° F. and maintaining the temperature of the aqueous liquor stream at or above 120° F. immediately prior to contact with the reverse osmosis membrane.
 9. The process of claim 8 where the Al(OH)₃ separated in step (c) is filtered to form a cake of Al(OH)₃ and a filtrate solution.
 10. The process of claim 9 where the Al(OH)₃ cake is recycled back to the alumina production facility for processing into Al₂O₃ and the filtrate solution is added to the clarifying step along with the mixture from step (f).
 11. The process of claim 6 where the filtering of the neutralized liquor stream in step (d) reduces the suspended solids level to a silt density index of <3.
 12. The process of claim 6 where the filtered neutralized liquor is at a temperature below 120° F. and is heat exchanged to increase the temperature to at or above 120° F.
 13. The process of claim 6 where a portion of the NaOH product solution is mixed with lime to form a lime slurry that is added to the crystallizer reactor.
 14. The process of claim 6 where a polymer solution is added to the clarification step (g). 